Despite successful control of HIV with combined antiretroviral therapy (cART), HIV-infected patients continue to harbor HIV in their brain and are at risk for HIV-associated neurocognitive disorders (HAND). This is in part due to the incomplete CNS penetrations by cART. However, cART toxicity to the brain tissue is also associated with HAND. Therefore, novel therapy is necessary to treat HIV in the CNS without causing toxicity. Recently, a new broadly neutralizing monoclonal antibody (mAb) against HIV, PGT121, was shown to achieve control of viral replication in the blood of simian human immunodeficiency virus (SHIV)-infected Rhesus monkeys. However, only limited quantity of PGT121 penetrates through the blood-brain-barrier (BBB); and it is also not known if PGT121 retains its functions in the CNS environment. Our overall goal is to reduce viral load in the CSN by re-engineer PGT121 for better penetration through the BBB. We will create a bispecific antibody where PGT121 is one arm of the antibody and the other arm binds the human transferrin receptor (HTfR) on the BBB to facilitate active transcytosis (PGT121/anti-HTfR Ab). The central hypothesis is that intravenous infusion with PGT121/anti-HTfR Ab will specifically target HIV in the CNS. Our Preliminary Data demonstrated distinct CSF IgG profile, detection of simian immunodeficiency virus (SIV) in the brain early after intrarectal infection in rhesus monkeys, and that less than 0.2% of intravenously infused PGT121 enters the CSF, which is below the threshold for efficacy. We posit that re-engineering PGT121 as a bispecific antibody (PGT121/anti-HTfR Ab) to efficiently across the BBB will treat HAND by reducing viral load without the cART-associated toxicity. We will test this hypothesis by pursuing three specific aims: 1.) Re-engineer mAb PGT121 as a bispecific Ab for improved CNS penetration. 2.) Determine preliminary efficacy and safety of bispecific PGT121/anti-HTfR Ab in SHIV-infected rhesus monkeys. 3.) Reduce SHIV replication in CNS reservoirs of infected rhesus monkeys with bispecific PGT121/anti- HTfR Ab. To achieve these, the Tan lab will continue our ongoing collaboration with the Reimann lab in generating and testing the bispecific antibody PGT121/anti-HTfR Ab in Aim 1. The non-human primate studies in Aims 2 and 3 were developed and will be carried out in collaboration between the Tan and Barouch labs. The development of novel tools such as the bispecific antibody will pave the way for the discovery of new therapeutics for eradication of HIV from the CNS. This proposed work will expand our understanding of antibody-mediated responses within the CNS, and create new therapeutic interventions targeting HIV in the CNS.